1. Field of the Invention
The present invention relates to a particulate filter for suppressing the emission of particulate matter contained in exhaust gas of an engine capable of a lean-burn operation, such as a diesel engine or a lean-burn gasoline engine.
2. Background Art
Exhaust gas from an engine capable of a lean-burn operation, such as a diesel engine or a lean-burn gasoline engine, contains therein particulate matter (PM) consisting primarily of carbonaceous components. With a view to reducing adverse environmental effects causes by the emission of such PM, a particulate filter is disposed in an exhaust passage of the engine to trap PM. The trapped PM will be gradually accumulated on the particulate filter in proportion to an operating time (engine operation time). Thus, the engine is designed to perform a control of burning the accumulated PM before an amount of accumulated PM reaches a predetermined value or more, to regenerate the particulate filter.
For example, as a technique of burning PM, there has been known one type of burning PM using a burner or a heater. In recent years, there has also been employed another type of burning hydrocarbon (HC) components contained in fuel and exhaust gas, by an oxidation catalyst disposed upstream of a particulate filter, to increase an exhaust gas temperature so as to promote burning of PM.
Generally, PM is self-ignited under an oxidation atmosphere at a temperature of slightly less than 700° C. It is known that this self-ignition temperature can be lowered by an action of a catalytic noble metal, such as platinum (Pt). Thus, some particulate filters are designed such that a catalyst layer containing, for example, Pt-supporting alumina particles, is wash-coated onto a surface of an exhaust gas channel allowing exhaust gas to pass therethrough, to allow PM to be burnt at a relatively low temperature. Further, with a view to effectively promoting ignition/burning of PM, it has been proposed a technique of incorporating, into a catalyst layer, an oxygen-absorbing/releasing material containing an oxide of cerium (Ce) and having an active-oxygen releasing capability, to utilize oxygen released from the oxygen-absorbing/releasing material, and a technique of incorporating, into a catalyst layer, an oxygen-ion conducting material containing an oxide of zirconium (Zr) and having an oxygen-ion conductivity, to utilize oxygen conducted through the oxygen-ion conducting material.
As an example of the above wash-coated particulate filter, JP 2006-326573A discloses a diesel particulate filter comprising a catalyst layer including a Ce—Zr based composite oxide which contains Ce, Zr, and a rare-earth element R except Ce, wherein a molar ratio of R/(Ce+Zr+R) is in the range of 2 to 11%.
JP 2007-054713A discloses a diesel particulate filter comprising a particulate matter oxidation catalyst including at least one of: a Zr-based composite oxide which contains Zr as a primary component, and a rare-earth metal except Ce and yttrium (Y); and a Ce-based composite oxide which contains Ce as a primary component, and a rare-earth metal except Ce or an oxide of an alkaline-earth metal.
JP 2007-083224A discloses a diesel particulate filter comprising a particulate matter oxidation catalyst including: a Zr-based composite oxide which contains Zr as a primary component, and a rare-earth metal except Ce; and alumina, wherein each of the Zr-based composite oxide and the alumina supports a catalytic noble metal thereon.
In an operation of burning PM, i.e., an operation of regenerating a particulate filter, it is necessary to allow exhaust gas to have a relatively high temperature. Thus, for example, following a main-injection for injecting fuel around a compression top dead center (i.e., top dead center of a compression stroke), a post-injection is performed at a non-ignition timing later than the compression top dead center, to add fuel to exhaust gas, and then the added fuel is burnt by an action of an oxidation catalyst so as to increase an exhaust gas temperature based on heat from the burning, to promote burning of PM. If a burning rate (i.e., burning velocity or speed) of PM (hereinafter referred to as “PM burning rate”) is low, it needs to take a long time for completely burning PM, which leads to an increase in fuel consumption for burning the PM, causing deterioration in fuel economy performance. In view of reducing the fuel consumption for burning PM, it is requited to further increase the PM burning rate as compared with those based on the catalysts disclosed in the JP 2006-326573A, the JP 2007-054713A and the JP 2007-083224A.
The JP 2006-326573A, the JP 2007-054713A and the JP 2007-083224A include a description that a higher PM burning rate can be obtained in an initial stage by increasing a content ratio of the rare-earth metal except Ce. However, it was considered that, if the content ratio of the rare-earth metal except Ce is increased, heat resistance deteriorates, and thereby the catalyst will be thermally damaged to cause a reduction in catalytic efficiency.
For reference, as an example of an exhaust gas purifying catalyst comprising a catalyst layer which includes a composite oxide containing Zr, the pamphlet of WO 06/046316A discloses an exhaust gas purifying catalyst comprising a catalyst support substrate, and a plurality of catalyst layers provided on a surface of the catalyst support substrate, wherein an uppermost one of the catalyst layers which defines a top surface thereof, includes a stabilized zirconia containing Zr, and a Ce—Zr composite oxide.
The catalyst disclosed in the pamphlet of WO 06/046316A is intended to enhance catalyst durability by means of incorporating the stabilized zirconia and the Ce—Zr composite oxide into the uppermost layer. Thus, this catalyst is incapable of promoting the burning of PM, and therefore cannot be directly applied to a particulate filter.